Voltage-dependent sodium and calcium currents in cultured parasympathetic neurones from rat intracardiac ganglia
1. Depolarization-activated Na+ and Ca2+ currents underlying the rising phase of the action potential in mammalian parasympathetic ganglion cells were investigated in voltage-clamped neurones dissociated from neonatal rat intracardiac ganglia and maintained in tissue culture. 2. A current component isolated by replacing intracellular K+ with Cs+ or arginine and adding 0.1 mM Cd2+ to the external solution was dependent on extracellular [Na+] and reversibly blocked in the presence of 300 nM tetrodotoxin (TTX). Peak amplitudes of Na+ currents elicited by step depolarization from a holding potential of -100 mV were 351 ± 18 pA/pF (140 mM extracellular Na+). 3. The sodium current-voltage (I-V) curve exhibited a threshold for activation at -40 mV and reached a maximum at 10 mV. The Na+ conductance increased sigmoidally with increasing depolarization reaching half-maximal activation at -25 mV, with a maximum slope corresponding to 7.5 mV per e-fold change in conductance. 4. During a maintained depolarization, Na+ currents turned on and then decayed (inactivated) with an exponential time course. The time constant of inactivation was voltage dependent decreasing from 0.85 ms at -20 mV to 0.3 ms at + 60 mV (23°C). The steady-state inactivation of the Na+ conductance was voltage-dependent with half-inactivation occurring at -61 mV and near-complete inactivation at -20 mV. Recovery from inactivation also followed an exponential time course with a time constant that increased at depolarized membrane potentials. 5. A voltage-and Ca2+-dependent current was isolated by replacement of intracellular K+ with either Cs+ or arginine and of extracellular Na+ with tetraethylammonium and the addition of TTX. Extracellular Ba2+ or Na+ (in the absence of external divalent cation) could substitute for Ca2+. Peak Ca2+ current increased with increasing extracellular [Ca2+] and above 10 mM (K(d) ~ 4 mM) approached saturation. The peak Ca2+ current density was 45 ± 4 pA/pF (2.5 mM -extracellular Ca2+). 6. The Ca2+ I-V relation exhibited a high threshold for activation (-20 mV) and reached a maximum at +20 mV. Changing the holding potential from -100 to -40 mV did not alter the I-V relationship. Peak Ca2+ conductance increased sigmoidally with increasing depolarization reaching half-maximal activation at -4 mV, with a maximal slope of 4 mV per e-fold change in Ca2+ conductance. 7. The kinetics of activation and inactivation of the Ca2+ current were voltage dependent and the time course of inactivation was fitted by the sum of two exponentials. The time to peak of the inward Ca2+ current decreased with increasing depolarization. With maintained depolarization the Ca2+ current slowly inactivated, by about 50% during a 400 ms pulse. Steady-state inactivation of the Ca2+ current was voltage dependent with half-inactivation occurring at -38 mV and complete inactivation at 0 mV. The rate of recovery from inactivation increased with hyperpolarization with both time constants reduced e-fold by a 60 mV hyperpolarization 8. Calcium currents were inhibited reversibly in a dose-dependent manner by external Cd2+, with half-maximal inhibition at 3.6 μM. The peak amplitude of the Ca2+ current was increased 21% by 5 μM Bay K 8644, and was inhibited by 5 μM nifedipine applied extracellularly. Raising the nifedipine concentration to ≥ 30 μM produced maximal inhibition of 67%. 9. The Ca2+ current was inhibited irreversibly by ~ 70% by bath application of a maximally effective dose of ω-conotoxin (ω-CGTX; 300 nM). The residual current in ω-CGTX was further inhibited by ~ 50% by 20 μM nifedipine. The ω-CGTX and dihydropyridine-resistant current was inhibited by Cd2+, suggesting that rat parasympathetic neurones contain at least three pharmacologically distinct types of calcium channel.